The present invention is related to wireless networks, and in particular to a method and apparatus for transmitting “mini-beacons” from an access point of a wireless local area network (WLAN) that provides sufficient information to associated or potentially-associating client stations, and that avoids the potential waste of bandwidth that would occur if only full-size beacons were broadcasted by the access point.
The examples herein will be described in terms of a wireless local area network that conforms to the IEEE 802.11 standard, but the invention is not restricted to such a network.
In IEEE 802.11-conforming wireless networks, at the MAC medium access control (MAC) level, beacon frames are sent out periodically by an access point (AP). One purpose of a beacon frame is to indicate to clients associated with the AP that the AP they are connected to is still working. Another purpose is to synchronize time in the basic service set (BSS) of the AP. Beacon frames convey information to the associated and associating clients as to the properties of the access point transmitting the beacons.
A new feature being developed for wireless devices is the ability for a single wireless device to act as multiple “virtual” access points, each for a different infrastructure wireless network. Thus, a single wireless device can act as an access point, also called a base station, for a plurality of infrastructure networks. Each infrastructure network is identified by a network identifier. In the language of the IEEE 802.11 standard, each infrastructure network is called a basic service set (BSS), and the network identifier is a basic service set identifier (BSSID). For an AP, the BSSID is the MAC address of the AP. Thus, a single wireless device can have a plurality of BSSIDs (MAC addresses). To a client station, a multiple BSSID-AP appears to be several distinct co-located APs.
Beacon frames can be quite large, especially for multiple BSSID APs, considering advanced features such as lists of SSIDs, (SSIDL, SSID lists), and especially in a busy environment. Furthermore, a single AP that act as a plurality of “virtual” APs needs to transmit a single beacon for each BSSID supported, and each such beacon may be large. Thus, beacon frames can consume significant bandwidth that could otherwise productively be used for data. This is exacerbated by the fact that beacon frames are transmitted at the lowest data rate for the basic service set (BSS).
As an example, consider a multiple-BSSID AP configured with 16 SSIDs, each of size 16 bytes. In such a case, the SSIDL element of a beacon frame is at least 16×16=256 bytes. The rest of the beacon might consist of around 100 bytes, for a total of 356 bytes for a single beacon frame. Suppose further that the beacon period is 100,000 μsec, and beacon frames are sent at a data rate of 1 Mbps. This results in about 3% of the transmission time, excluding medium access times, dedicated to transmitting beacon frames. If one scales this up to multiple BSSID's on a single channel, then the relative time used on beacon overhead can become even more significant. For example, for 16 BSSIDs, the time taken by beacons is more than 45%.
Furthermore, as more and more features and capabilities are added to wireless networks, the size of beacon frames keeps increasing as more information is added to such beacon frames.
Thus there is a need in the art for a mechanism other than full IEEE 802.11 MAC beacon frames, in order to reduce the bandwidth used by beacon frames.
Recently, a proposal for “Scheduled Autonomous Probe Response” (SAPR) was made by Motorola, inc. in an IEEE 802.11 Group k (IEEE 802.11k) Wireless LAN meeting. See “IEEE 802.11 04/1010: Proposal and Normative Text for a Scheduled Autonomous Probe Response Generation Function,” Date: Sep. 9, 2004, authors Steve Emeott, et al. The idea is to reduce passive scan latency. The SAPR generation function allows an AP to autonomously transmit Probe Response frames at periodic intervals. The SAPR interval is indicated by a SAPR subfield included in a SAPR information element within Beacon and Probe Response frames. The presence of scheduled, autonomous Probe Response frames with the SAPR information element allows stations to rapidly take measurements and find neighbor APs using passive scanning. Thus, the idea is to reduce the amount of time required for passive scanning clients to “find” an AP. Thus, the purpose of SAPR frames is to increase interactivity. Such SAPR frames are for the purpose of passive scanning (measuring RSSI), e.g., by clients of the AP. SAPR frames are also called Gratuitous Probe Response-(GPR) frames.
There still remains a need in the art for a mechanism to provide essential information that is in beacon frames to associated or associating client stations, while decreasing the amount of bandwidth used compared to using full, potentially large beacon frames. This potentially frees up bandwidth for non-management traffic.